The present invention relates to a white balance adjusting device suitable for a color video camera, and more particularly to a device which automatically adjusts a white balance in accordance with a change of a color temperature.
An internal signal measurement type automatic tracking auto-white balancing device which processes a signal from an image pickup device by a signal processing circuit to detect a change in a color temperature for an illuminating light source based on the processed signal and controls gains of red (R) signal and blue (B) signal circuits of a chrominance signal system of a camera in accordance with the detected signal to automatically adjust the white balance, has been known. Examples of such devices are disclosed in JP-A-No. 58-142693, JP-A-No. 59-189793 and U.S. Pat. No. 3,786,177. A principle of the white balance device of this type is based on the fact that a screen average of differential chrominance signals R-G (or R-Y.sub.L, where Y.sub.L is a low frequency component of a brilliance signal), B-G (or B-Y.sub.L) signals or R-B signals produced by imaging a general object is in almost all cases an approximation to a value derived from a non-colored area of the general object. Based on this presumption, the screen average of the differential chrominance signals is zero if the white balance matches and the average signal changes in accordance with the change of the color temperature as the illumination to the object changes. Thus, the change is detected to control the gains of the R and B signal gain control circuits so that the white balance is adjusted. In JP-A-No. 58-142693, the R-G and B-G signals are produced from three principal color signals and they are filtered and negatively fed back in the R and B gain control circuits so that the chrominance signals are rendered zero in order to attain the white balance adjustment. In JP-A-No. 59-189793 and U.S. Pat. No. 3,786,177, in addition to the presumption of equivalence to white, an experimental fact that it is rare that a general object which does not include a white area is imaged is also taken into consideration. Thus, the white area is extracted from the object and the differential chrominance signal such as R-B signal of the white area is detected. When the average signal of differential chrominance signal is used, a probability of determination of equivalence to non-color increases and more correct control voltage can be produced for the change of illumination of the object, that is, the change of color temperature.
However, for an object which does not satisfy the above presumption, for example, single green color (monocolor) object such as wide green, field or a number of plants, it is impossible to determine that the average signal is equivalent to non-color. As a result, under such an imaging condition, the prior art device may produce an incorrect white balance control signal or cannot detect white and the white balance control does not operate and color reproducibility is degraded. The degradation of the color reproducibility increases as an area of monocolor on a monitor screen increases. The prior art device needs two separate negative feedback control circuits for the R-signal channel and B-signal channel, or it needs memories for storing R, G and B signals one for each of typical imaging light sources such as tungsten light and solar light, or it needs level comparators for R-G and B signals for detecting a white area and a memory for storing previous white balance control information until new control is started after the detection of the white area. As a result, a circuit scale is large and versatile. Where the white balance is adjusted by feed-forward control, it becomes difficult in such a design to have tracking accuracy and circuit stability, such as, between a control signal and a controlled amount.